feat(phase0): complete GC1 feasibility and GC2 shared format contracts

- Record user evidence for directory fallback to complete GC1 (10/10 checks passed)

- Expand Format Specification 0.1 to Revision 0.2 with normative shared contracts

- Author JSON Schema Draft-07 at schema/xzbt-0.1.schema.json

- Implement zero-dependency semantic validator at tools/validate-exhibit.mjs

- Create 12-case conformance fixture suite and automated test runner (12/12 passing)

- Update implementation status, verification gates, and gap closure decisions

- Add devlog entry covering stall recovery and Phase 0 current state

test/fixtures/gc2/invalid-reference-missing.xzbt

@@ -0,0 +1,19 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "missing-ref-study",
+    "name": "Invalid Reference Missing Study"
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 0.5
+    }
+  },
+  "bindings": [
+    {
+      "from": "parameters.non_existent_param",
+      "to": "parameters.activity"
+    }
+  ]
+}